[swift-evolution] [Pitch] Improving capturing semantics of local functions

[Slava Pestov]

> On Nov 16, 2017, at 4:38 PM, Howard Lovatt <howard.lovatt at gmail.com> wrote:
> 
> When the user writes:
> 
>     let increment: <T>(T) throws -> T where T: Numeric = { $0 + 1 }
>     increment(1) // 2
>     increment(1.1) // 2.1

This means that ‘increment’ is a *value* with a generic function type. Presumably you want to pass generic closures as function parameters and results too. This is called higher-rank polymorphism and it introduces considerable complexity in type checking and code generation.

> Compiler issues global struct as above. Then:
> 
>     let _int_increment = _Function1__T1__T1__T1__E__Numeric<Int>({ $0 + 1 })
>     try _int_increment.call(1) // 2
>     let _double_increment = _Function1__T1__T1__T1__E__Numeric<Double>({ $0 + 1 })
>     try _double_increment.call(1.1) // 2.1

What if I do,

let array = [increment]

What is the type of ‘array’?

Slava

> 
> The more restrictive form that you suggest (I think this is what you mean anyway) of only allowed locally, not globally, is easier to name mangle, you just need a unique name, nothing about the name needs to be canonical. This would be similar to local functions at present and would be useful (though I am not sure how many local *generic* functions there are).
> 
> 
>   -- Howard.
> 
> On 17 November 2017 at 10:47, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
> 
> 
>> On Nov 16, 2017, at 3:07 PM, Howard Lovatt via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Where I am proposing a change is that if a closure with generic arguments is encountered it is transformed into the equivalent struct and the struct is typed as it currently is (or if there is a better implementation something equivalent to this), therefore zero change to the type system.
> 
> Since we already have local functions that can capture values and be generic, there’s no need to implement a new mechanism for name mangling or handling of captures.
> 
>> 
>> The changes proposed are a transformation into a struct and name mangling, e.g.:
>> 
>>     let increment: <T>(T) throws -> T where T: Numeric = { $0 + 1 }
>>     let increment = { <T>(n: T) throws -> T where T: Numeric in n + 1 }
>>     let increment: <T>(T) throws -> T where T: Numeric = { <T>(n: T) throws -> T where T: Numeric in n + 1 }
> 
> It sounds like what you’re proposing is essentially a new surface syntax for local functions — since a generic closure would not be a first class value, it could not appear anywhere except for the right hand side of a let binding, right?
> 
> Slava
> 

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